<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="text" id="i1">
<input type="text" id="i2">
<input type="button" value="+" onclick="f(1)">
<input type="button" value="-" onclick="f(2)">
<input type="button" value="*" onclick="f(3)">
<input type="button" value="/" onclick="f(4)">
<h3>
  结果：<span></span>
</h3>
<script>
    let i1=document.querySelector("#i1");
    let i2=document.querySelector("#i2");
    let s=document.querySelector("span")
    function f(num) {
        if (isNaN(i1.value)||isNaN(i2.value)){
            s.innerText="输入错误";
            return
        }
        switch (num){
            case 1:  s.innerText=i1.value*1+i2.value*1;break;
            case 2: s.innerText=i1.value*1-i2.value*1;break;
            case 3: s.innerText=i1.value*1*i2.value*1;break;
            case 4: s.innerText=i1.value*1/i2.value*1;break;

        }
    }

    // function f2() {
    //     if (isNaN(i1.value)||isNaN(i2.value)){
    //         s.innerText="输入错误";
    //         return
    //     }
    //     s.innerText=i1.value*1-i2.value*1;
    // }
    // function f3() {
    //     if (isNaN(i1.value)||isNaN(i2.value)){
    //         s.innerText="输入错误";
    //         return
    //     }
    //     s.innerText=i1.value*1*i2.value*1;
    // }
    // function f4() {
    //     if (isNaN(i1.value)||isNaN(i2.value)){
    //         s.innerText="输入错误";
    //         return
    //     }
    //     s.innerText=i1.value*1/i2.value*1;
    // }
</script>
</body>
</html>